During the drilling and completion of oil and gas wells, it may be necessary to engage in ancillary operations, such as monitoring the operability of equipment used during the drilling process or evaluating the production capabilities of formations intersected by the wellbore. For example, after a well or well interval has been drilled, zones of interest are often tested to determine various formation properties such as permeability, fluid type, fluid quality, formation temperature, formation pressure, bubblepoint, formation pressure gradient, mobility, filtrate viscosity, spherical mobility, coupled compressibility porosity, skin damage (which is an indication of how the mud filtrate has changed the permeability near the wellbore), and anisotropy (which is the ratio of the vertical and horizontal permeabilities). These tests are performed in order to determine whether commercial exploitation of the intersected formations is viable and how to optimize production.
Wireline formation testers (WFT) and drill stem testers (DST) have been commonly used to perform these tests. The basic DST tool consists of a packer or packers, valves, or ports that may be opened and closed from the surface, and one or more pressure-recording devices. The tool is lowered on a work string to the zone to be tested. The packer or packers are set, and drilling fluid is evacuated to isolate the zone from the drilling fluid column. The valves or ports are then opened to allow flow from the formation to the tool for testing while the recorders chart the pressure transients. A sampling chamber traps formation fluid at the end of the test. WFTs generally employ the same testing techniques but use a wireline to lower the test tool into the borehole after the drill string has been retrieved from the borehole. The WFT typically uses packers also, although the packers typically isolate a much smaller borehole area, compared to DSTs, for more efficient formation testing. In most cases, the WFT do not use conventional packers but rather probe devices that isolate only a small circular region on the borehole wall.
The WFT probe assembly engages the borehole wall and acquires formation fluid samples. The probe assembly may include an isolation pad to engage the borehole wall. The isolation pad seals against the formation and around a hollow probe, which places an internal cavity in fluid communication with the formation. This creates a fluid pathway that allows formation fluid to flow between the formation and the formation tester while isolated from the borehole fluid.
In order to acquire a useful sample, the probe must stay isolated from the relative high pressure of the borehole fluid. Therefore, the integrity of the seal that is formed by the isolation pad is critical to the performance of the tool. If the borehole fluid is allowed to leak into the collected formation fluid, a non-representative sample and pressure measurement will be obtained and the test will have to be repeated.
Examples of isolation pads and probes used in WFTs can be found in Halliburton's DT, SFTT, SFT4, and RDT tools. Isolation pads that are used with WFTs are typically rubber pads affixed to the end of the extending sample probe. The rubber is normally affixed to a metallic plate that provides support to the rubber as well as a connection to the probe. These rubber pads are often molded to fit within the specific diameter hole in which they will be operating.
With the use of WFTs and DSTs, the drill string with the drill bit must first be retracted from the borehole. Then, a separate work string containing the testing equipment, or, with WFTs, the wireline tool string, must be lowered into the well to conduct secondary operations.
DSTs and WFTs may also cause tool sticking or formation damage. Sticking occurs when the tool's body contacts the borehole for an extended period of time. A seal is formed and the differential pressure between the borehole and the formation draws the tool in close contact with the formation and causes the tool to be stuck. Formation damage occurs due to the extended periods the borehole is in the presence of hydrostatic pressures causing drilling fluid invasion to continue. There may also be difficulties of running WFTs in highly deviated and extended reach wells. When sticking or tight sections are encountered only the wireline can be used to retrieve the stuck tool. WFTs also do not have flowbores for the flow of drilling mud that helps prevent sticking. WFTs are also not designed to withstand drilling loads such as torque and weight on bit.
Further, the formation pressure measurement accuracy of drill stem tests and, especially, of wireline formation tests may be affected by mud filtrate invasion and mudcake buildup because significant amounts of time may have passed before a DST or WFT engages the formation after the borehole has been drilled. Mud filtrate invasion occurs when the drilling mud fluids displace formation fluid. Because the mud filtrate ingress into the formation begins at the borehole surface, it is most prevalent there and generally decreases further into the formation. When filtrate invasion occurs, it may become impossible to obtain a representative sample of formation fluid or, at a minimum, the duration of the sampling period must be increased to first remove the drilling fluid and then obtain a representative sample of formation fluid. Mudcake buildup occurs when any solid particles in the drilling fluid are plastered to the side of the wellbore by the circulating drilling mud during drilling. This mudcake helps to isolate and impede the invasion. Frequently, the mud filtrate carries particles into the formation pore spaces, significantly reducing the permeability near the borehole surface. Thus there may be a “skin effect”. Because formation testers' pressure transient can only extend relatively short distances into the formation, the measurement of formation permeability can be distorted. The skin effect also reduces the flow rate into the tool thereby impeding the tester's ability to obtain a representative sample of formation fluid. While the mudcake also acts as a region of reduced permeability adjacent to the borehole, it is essential to reducing filtrate invasion. Essentially, the mudcake is the primary seal and aids in obtaining accurate reservoir pressure measurements and formation samples. Normally the mudcake is easily penetrated by WFT probes and zones isolated with inflatable packers. However, the internal skin can reduce the tester's abilities.
Another testing apparatus is the formation tester while drilling (FTWD) tool. Typical FTWD formation testing equipment is suitable for integration with a drill string during drilling operations. Various devices or systems are used for isolating a formation from the remainder of the borehole, drawing fluid from the formation, and measuring physical properties of the fluid and the formation. Fluid properties, among other items, may include fluid compressibility, flowline fluid compressibility, density, viscosity, resistivity, composition, and bubblepoint. For example, the FTWD may use a probe similar to a WFT that extends to the formation and a small sample chamber to draw in formation fluid through the probe to test the formation pressure.
To perform a test, the drill string is stopped from rotating and moving axially and the test procedure is performed. The FTWD tool is positioned over the formation to form a seal between the tool and the formation, thereby isolating the formation from the remainder of the borehole. Fluid is then drawn from the formation into a sample chamber contained within the tool. The sample chamber may be formed by a cylinder within the tool and the sealed formation. The volume of the sample chamber may be increased or decreased by translating a piston within the cylinder. To break through the mudcake seal over the formation and to initiate fluid flow from the formation into the sample chamber, the piston is translated to increase the volume of the sample chamber, thereby lowering the fluid pressure inside the sample chamber. This process is referred to as drawdown. After drawdown is completed, formation fluid continues to flow into the sample chamber. The pressure of fluid inside the sample chamber is monitored and recorded until it reaches the formation pressure. The length of time required to complete this buildup process may be lengthy, causing the loss of valuable drilling rig time. This may be particularly so in low permeability or low mobility formations. For example, less than 1 md/cp.